Transparent color of a Glyph

 
When you place a glyph (bitmap-picture) on a button, the color of the pixel in the picture's lower-left corner will be used as the transparent color. All the pixels of that color will be transparent, the button's surface will be visible at these spots.

If you don't want any transparent parts on your button bitmap, make sure to use a color in the lower-left corner that is not used anywhere else in the bitmap.